Lens for optical data storage system

ABSTRACT

An optical data storage system includes an optical disc which stores information in optically readable format on a data surface. A slider is provided proximate the data surface of the optical disc and is coupled to an actuator for selectively positioning the slider relative to the data surface. A Solid Immersion Lens couples to the slider and is arranged to couple light to the data surface of the optical disc. The Solid Immersion Lens is non-hemispherical.

This is a Divisional patent application of U.S. Ser. No. 08/911,556,filed Aug. 14, 1997, which is based on Provisional Application Ser. No.60/039,934 filed on Mar. 10, 1997.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical disc data storagesystems. More specifically, the present invention relates to opticalstorage systems which utilize a Solid Immersion Lens (SIL) for focusinglight onto a data surface of the disc.

Optical data storage disc systems are a promising technology for storinglarge quantities of data. The data is accessed by focusing a laser beamonto a data surface of the disc and analyzing light reflected from ortransmitted through the data surface.

In general, in optical storage systems, data is in the form of markscarried on the surface of the disc which are detected using thereflected laser light. There are a number of different optical disctechnologies which are known in the industry. For example, compact discsare currently used to store digital data such as computer programs ordigitized music. Typically, compact discs are permanently recordedduring manufacture. Another type of optical system is a write-onceread-many (WORM) system in which a user may permanently writeinformation onto a blank disc. Other types of systems are erasable, suchas phase change and magneto-optic (M-O) systems. Phase change systemsdetect data by sensing a change in reflectivity. M-O systems read databy measuring the rotation of the incident light polarization due to thestorage medium.

The above systems require a beam of light to be focused onto a datasurface of a disc and recovering the reflected light. Storage density isdetermined not only be the size of the markings on the data surface, butalso by the size of the beam focused on the surface (i.e. resolution).One type of optical element which can be used in conjunction with anobjective lens to reduce the ultimate spot size of the light beam is aSolid Immersion Lens or SIL. A SIL reduces the beam spot size by virtueof the wavelength reduction which occurs when light is inside anoptically dense medium. The SIL is positioned very close to the datasurface of the disc and couples light to the disc surface via evanescentwave effects. This is often referred to as the "near-field" regime. Theuse of SILs for data storage is described in U.S. Pat. No. 5,125,750 toCorle et al. which issued Jun. 30, 1992 and in U.S. Pat. No. 5,497,359to Mamin et al. which issued Mar. 5, 1996. In these optical systems, alaser beam is focused onto the SIL using an objective lens. The SIL ispreferably carried on a slider and the slider is positioned close to thedisc data surface.

In optical storage systems, it is typically preferred to move the beambetween adjacent tracks without moving the slider. This allows moreaccurate and faster tracking control. However, the SILs of prior artoptical systems have been designed to optimize spot size, without regardto off-axis performance.

SUMMARY OF THE INVENTION

The present invention includes an optical data storage system having anoptical disc for storing information in an optically readable format ona data surface. A slider is positioned adjacent to the data surface ofthe optical disc and is coupled to an actuator which selectivelypositions the slider relative to the data surface. The system alsoincludes a light source. A lens for near-field coupling light to thedata surface is mounted on the slider. The lens comprises a SolidImmersion Lens (SIL) having a non-hemispherical surface directed towardthe light source and an opposing surface. The opposing surface ispositioned sufficiently close to the data surface for near fieldcoupling to the data surface. A separate objective lens is positionedbetween the SIL and the light source. The objective lens and the SIL areselected to improve off-axis performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing an optical storage system using aSIL in accordance with one embodiment of the present invention.

FIG. 2 is a cross-sectional view of a prior art Solid Immersion Lens.

FIG. 3 is a cross-sectional view of a Solid Immersion Lens using a lenscap in accordance with another embodiment of the present invention.

FIG. 4 is a cross-sectional view of a slider having a Solid ImmersionLens in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to optical data storage systems. Morespecifically, the present invention relates to lenses which are used foroptically coupling to a data surface of a storage medium using theoptical "near-field." One lens for such coupling is a Solid ImmersionLens (SIL). The use of Solid Immersion Lenses for optical storagesystems is described in U.S. Pat. Nos. 5,125,750 and 5,497,359.

The Solid Immersion Lens has been proposed as a means of increasing theoptical storage areal storage density by moving from the far-fieldrecording regime to the near-field recording regime. In far-fieldrecording, the spacing between the nearest optical component and therecording medium is substantially greater than the wavelength of therecording light. In contrast, in near-field recording the separation ismuch less than the wavelength of the recording light. In order tomaximize areal storage density, it is necessary to minimize therecording spot. It can be shown that the recording spot of anobjective-Solid Immersion Lens pair is related as:

    spot size α (λ/NA)                            Eq. 1

where λ is the wavelength of the light used to generate the spot and NAis the numerical aperture of the objective lens. In the absence of theSIL, λ=λ_(o) where λ_(o) is the wavelength of light in air. When thelight spot is inside the SIL, then λ=λ_(o) /n wherein n is the index ofrefraction of the SIL. Hence, by selecting a SIL material with a largeindex of refraction, the spot size can be reduced by 1/n since:

    λ=λ.sub.o /n                                 Eq. 2

in the SIL.

Since the areal storage density varies roughly according to the inverseof the spot size squared, it can be shown that:

    Areal Density α (spotsize).sup.-2 α (NA/λ).sup.2 α n.sup.2                                                   Eq. 3

Thus, it is beneficial to work with high index materials. Typicaloptical glasses have an index in the range of 1.5-1.9. Materials likesapphire, zirconia, and diamond have indexes which range from roughly1.7-3. Hence, the areal density achievable with a SIL can be increasedover that achievable with a single recording objective anywhere from2.25 to 9. From a pure recording areal density perspective, suchmaterials as zirconia and diamond are favored by virtue of their highindex of refraction. However, one major problem with using materialslike zirconia or diamond is their cost. When trying to volumemanufacture objective-SIL lens pairs to increase optical storagecapacity, this cost can have tremendous system implications.

The present invention provides a Solid Immersion Lens which, as desired,may be produced without incurring the cost of the exotic materialstypically used in prior art Solid Immersion Lenses. The lens is easilyfabricated and can be coupled to or integrated on a slider of the typeused in disc storage systems. The invention recognizes that a desiredspot size may be obtained with a non-hemispherical lens by fabricating alensing system with the appropriate λ/NA. Further, the invention allowsthe Field-Of-View (FOV) of the lensing system to be optimized for aparticular configuration. This is particularly important where it isdesired to move the spot between adjacent tracks, without moving theslider.

FIG. 1 is a simplified illustration of an optical recording system 10employing a Solid Immersion Lens in accordance with the presentinvention. System 10 includes optical disc 12 having a data surfacewhich carries optically encoded information. Disc 12 rotates aboutspindle 14 and is driven by a spindle motor 16 mounted on base 18. Aslider 20 is positioned proximate disc 12 and is coupled to an actuator22 which includes armature 24 and actuator motor 26 which couples tobase 18. Slider 18 includes optical elements 30 in accordance with thepresent invention. An optical source/sensor apparatus 32 is opticallycoupled to elements 30 through mirror 33. A controller 34 couples toapparatus 32, mirror 33, actuator 26 and data bus 36 and is used forcontrolling operation of system 10.

During operation, disc 12 rotates and slider 20 is positioned radiallyover the data surface of disc 12 using actuator 22. Controller 34controls the position of slider 20, whereby information may be read from(and in some embodiments written to) the data surface of disc 12 usingoptical source/sensor apparatus 32 and optical elements 30. Precisecontrol of spot position is achieved by controller 34 scanning the spotacross the disc surface over several tracks. For example, this could beby moving mirror 33. The precise configuration of apparatus of 32 may beselected based upon the type of storage system 10 employed. In general,source/sensor apparatus 32 includes an optical source which directslight toward optical elements 30 for illuminating the data surface ofdisc 12. Light is reflected back through optical elements 30 from thedata surface of disc 12 for detection by apparatus 32. Controller 34senses the reflected signal which is converted to data for transmissionon data bus 36.

One aspect of the present invention includes the recognition that priorart objective-SIL lens pairs utilize two important features in achievingspot reduction and the corresponding areal density increase as set forthin Equations 1-3 above. First, the spot is typically generated at thebottom of the SIL and spot is kept inside the SIL. This allows the SILmaterial to reduce the effective wavelength of the light by the index ofrefraction of the SIL material in accordance with Equation 2. Secondly,the objective-SIL lens pair maintains the effective numerical apertureof the light inside of the SIL. This is equivalent to maintaining thesolid angle of the cone of light from the objective lens which generatesthe spot inside the SIL. FIG. 2 illustrates these features and shows aprior art SIL 50 receiving light rays 52 from an objective lens. A spot54 is formed at the bottom of SIL 50 and within SIL 50.

However, if the light source is moved off axis, such occurs whenscanning adjacent data tracks, the spot size rapidly degrades. This isillustrated with rays 52' and spot 54'. As illustrated, rays 52' are notnormal to the surface of SIL 50 and are therefore not optimallyfocussed. This causes spot 54' to "smear," thereby reducing thedefinition and the density of the system.

Using the above two features in one embodiment, the invention providesan objective--SIL lens pair for near field recording utilizing anon-hemispherical SIL. As used herein, "non-hemispherical" includes anylens shape which does not include a hemispherical portion (i.e., andtherefore does not include a super-hemisphere), such as an asphericallens or a lens of less than a hemisphere. In one aspect of theinvention, the new lens pair is designed to maintain λ/NA to besubstantially the same as the λ/NA for the prior art lens beingreplaced. Further, as the invention is not limited by the designconstraints of prior art hemispherical SILs, the lens pair can improvethe field-of-view (FOV) in comparison to prior art systems. This allowsa greater range of placement options when positioning the light sourcerelative to objective lens and when steering the spot with the mirror33. In another aspect of the invention shown in FIG. 3, a SIL isprovided having a surface which is less than a hemisphere.

FIG. 3 is a cross-sectional view of a SIL lens 70 in accordance with onepreferred embodiment. Lens 70 includes a plano-convex asphere lens cap72 mounted on optically transparent substrate 64. The embodiment of FIG.3 is particularly advantageous because plano-convex asphere lens cap 72does not need to have the same high index of refraction material as usedfor substrate 64. Thus, lens cap 72 may be fabricated using lessexpensive materials and materials which are more easily machined ormolded such as glass. Cap 72 is easily fabricated through a moldingprocess and thus offers significant cost savings over the high index ofrefraction full hemispherical lenses offered in the prior art. Further,one aspect of the invention includes fabrication of any desirednon-hemispherical lens for the lens cap and the particular lens is notlimited to the plano-convex asphere lens shown. This allows the opticalcharacteristics of the SIL to be tailored as desired for specificapplications, system characteristics, objective lens, etc. For example,the field of view may be adjusted as desired.

FIG. 4 is a simplified side cross sectional view of slider 20 showingoptical elements 30 in accordance with one embodiment of the presentinvention. Slider 20 is shown proximate the data surface 48 of opticaldisc 12. Elements 30 include SIL 70 and an objective lens 80. The SIL 70can be a separate entity mounted on slider 20 or integrally combinedwith slider 20. Objective lens 80 is supported by a mounting member 82above SIL 50. Mount 82 positions objective lens 80 such that thedistance between objective lens 80 and SIL 70 is appropriate for theirfocal lengths. Note that in the embodiment of FIG. 4, substrate 64 isformed to provide slider 20.

FIG. 4 also illustrates an important feature of the invention, theoff-axis performance of optical elements 30. As shown in FIG. 4, on-axisrays 90 provide a focussed spot 92 within substrate 64 for coupling tosurface 48 using the near-field. However, to access an adjacent track,controller 34 need only scan the spot to provide off-axis beam 90'. Thisyields a focussed spot 92' which is offset from spot 92 allowing accessto adjacent data without repositioning slider 20. Further, this benefitis achieved without undue manufacturing costs. The invention includesthe recognition that the top surface of the SIL can have a non-sphericalprofile. This is particularly advantageous when coupled with a separateobjective lens which provides a total of three separate surfaces whichcan be controlled to optimize spot size servoability. This can beachieved using known lens modeling techniques.

The non-hemispherical lens of the present invention is preferably costeffectively produced using the same processes used to produce theobjective lens, for example objective lens 80. In one preferredembodiment, the plano surface of the plano-convex asphere lens ispreferred due to reduced aberration and ease of assembly. However, theinvention is not limited to such a design. Furthermore, in someembodiments, the optical characteristics of the objective lenses arepreferably designed to properly match the SIL. One aspect of theinvention includes a SIL using any "non-hemispherical" lens.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, the lens cap may be fabricatedusing any appropriate technique and bonded to or formed with thesubstrate as desired. The lens cap is not limited to the specificembodiments set forth herein. Furthermore, the substrate may takedifferent forms as desired and is not limited to the shapes set forthherein. Further, the description herein refers generally to "optical"and "light," however, these terms are intended to include other,non-visible radiation such as infrared ultraviolet and beyond. The servomechanism may be any appropriate device or technique and is not limitedto the mirror arrangement set forth herein.

What is claimed is:
 1. A method of reading or writing on an optical discin an optical disc storage system, comprising:moving a slider to aposition adjacent the storage disc; generating a light beam; directingthe light beam toward an objective lens at a first angle; directing thelight beam from the objective lens toward a Solid Immersion Lens,generating a first spot, and coupling the first spot to a first locationon the storage disc; directing the light beam toward the objective lensat a second angle; and directing the light beam from the objective lensand generating a second spot, the second spot spaced apart from thefirst spot, and coupling the first spot to a second location on thestorage disc.
 2. The method of claim 1 wherein the steps of directingthe light beam toward the objective lens include directing the lightbeam toward a mirror and controlling an angle between the light beam anda surface of the mirror.
 3. The method of claim 1 wherein the first andsecond spots are located within the Solid immersion Lens.
 4. The methodof claim 1 wherein the first and second spots are located below theSolid Immersion Lens and adjacent the storage disc.
 5. The method ofclaim 1 wherein the first and second spot locations correspond to datatracks on the storage disc.